Vacuum processors for processing a substrate (e.g., etching materials from or depositing materials onto the substrate), typically include ports respectively connected to a vacuum pump and one or more sources of ionizable, processing gases. The gas is excited to a plasma in a processing chamber by an electrical source including a reactance responsive to a first alternating current (A/C) source, typically a radio frequency (RF) source or microwave source. If the source is an RF source, the reactance is either a coil for supplying magnetic and electric fields t the chamber interior via a dielectric window or a parallel plate capacitive arrangement for supplying an electrostatic field to the processing chamber.
The substrate, which is typically a semiconductor wafer, dielectric sheet, or metal plate, is clamped in place on a mounting chuck that frequently includes an electrode covered by a dielectric. DC voltage is typically applied to the electrode to provide an electrostatic clamping force to hold the substrate in situ on the mounting chuck. To accelerate ions in the plasma to the substrate, a second NC source may be connected to the electrode or to another electrode by way of a matching network (e.g., RF bias).
During processing, anomalies occur in the processing chamber pressure and gas flow rates. The anomalies affect the plasma impedance as coupled to the excitation reactance and the RF bias electrode. As a result, processing rate non-uniformities can develop.
Therefore, the inventors have provided improved methods and apparatus for reducing and/or eliminating process rate non-uniformities in a plasma enhanced substrate processing chamber.